use latch::{LatchProbe, SpinLatch};
#[allow(unused_imports)]
use log::Event::*;
use job::StackJob;
use registry::{self, WorkerThread};
use std::any::Any;
use unwind;

#[cfg(test)]
mod test;

/// The `join` function takes two closures and *potentially* runs them
/// in parallel. It returns a pair of the results from those closures.
///
/// Conceptually, calling `join()` is similar to spawning two threads,
/// one executing each of the two closures. However, the
/// implementation is quite different and incurs very low
/// overhead. The underlying technique is called "work stealing": the
/// Rayon runtime uses a fixed pool of worker threads and attempts to
/// only execute code in parallel when there are idle CPUs to handle
/// it.
///
/// When `join` is called from outside the thread pool, the calling
/// thread will block while the closures execute in the pool.  When
/// `join` is called within the pool, the calling thread still actively
/// participates in the thread pool. It will begin by executing closure
/// A (on the current thread). While it is doing that, it will advertise
/// closure B as being available for other threads to execute. Once closure A
/// has completed, the current thread will try to execute closure B;
/// if however closure B has been stolen, then it will look for other work
/// while waiting for the thief to fully execute closure B. (This is the
/// typical work-stealing strategy).
///
/// ### Warning about blocking I/O
///
/// The assumption is that the closures given to `join()` are
/// CPU-bound tasks that do not perform I/O or other blocking
/// operations. If you do perform I/O, and that I/O should block
/// (e.g., waiting for a network request), the overall performance may
/// be poor.  Moreover, if you cause one closure to be blocked waiting
/// on another (for example, using a channel), that could lead to a
/// deadlock.
///
/// ### Panics
///
/// No matter what happens, both closures will always be executed.  If
/// a single closure panics, whether it be the first or second
/// closure, that panic will be propagated and hence `join()` will
/// panic with the same panic value. If both closures panic, `join()`
/// will panic with the panic value from the first closure.
pub fn join<A, B, RA, RB>(oper_a: A, oper_b: B) -> (RA, RB)
    where A: FnOnce() -> RA + Send,
          B: FnOnce() -> RB + Send,
          RA: Send,
          RB: Send
{
    registry::in_worker(|worker_thread| unsafe {
        log!(Join { worker: worker_thread.index() });

        // Create virtual wrapper for task b; this all has to be
        // done here so that the stack frame can keep it all live
        // long enough.
        let job_b = StackJob::new(oper_b, SpinLatch::new());
        let job_b_ref = job_b.as_job_ref();
        worker_thread.push(job_b_ref);

        // Execute task a; hopefully b gets stolen in the meantime.
        let result_a = match unwind::halt_unwinding(oper_a) {
            Ok(v) => v,
            Err(err) => join_recover_from_panic(worker_thread, &job_b.latch, err),
        };

        // Now that task A has finished, try to pop job B from the
        // local stack.  It may already have been popped by job A; it
        // may also have been stolen. There may also be some tasks
        // pushed on top of it in the stack, and we will have to pop
        // those off to get to it.
        while !job_b.latch.probe() {
            if let Some(job) = worker_thread.take_local_job() {
                if job == job_b_ref {
                    // Found it! Let's run it.
                    //
                    // Note that this could panic, but it's ok if we unwind here.
                    log!(PoppedRhs { worker: worker_thread.index() });
                    let result_b = job_b.run_inline();
                    return (result_a, result_b);
                } else {
                    log!(PoppedJob { worker: worker_thread.index() });
                    worker_thread.execute(job);
                }
            } else {
                // Local deque is empty. Time to steal from other
                // threads.
                log!(LostJob { worker: worker_thread.index() });
                worker_thread.wait_until(&job_b.latch);
                debug_assert!(job_b.latch.probe());
                break;
            }
        }

        return (result_a, job_b.into_result());
    })
}

/// If job A panics, we still cannot return until we are sure that job
/// B is complete. This is because it may contain references into the
/// enclosing stack frame(s).
#[cold] // cold path
unsafe fn join_recover_from_panic(worker_thread: &WorkerThread,
                                  job_b_latch: &SpinLatch,
                                  err: Box<Any + Send>)
                                  -> !
{
    worker_thread.wait_until(job_b_latch);
    unwind::resume_unwinding(err)
}
